[Downloads] Cleanup DownloadResourceHandler

* Avoid duplicate assignments.
* Don't calculate whether strong validators are present in an HTTP
  response. A spec compliant implementation of this determination is
  already present elsewhere.

BUG=7648

Review URL: https://codereview.chromium.org/66993008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236825 0039d316-1c4b-4281-b951-d872f2087c98
Changing exit to raise for not overridden test, to match the change made in 83043003

BUG=316397

Review URL: https://codereview.chromium.org/83703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236826 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236826